1. Field of the Invention
The present invention relates to a power amplifying circuit used for electronic apparatuses of a low power consumption type such as a mobile communication equipment.
2. Description of the Related Art
A conventional power amplifying circuit used in a low power consumption type electronic apparatus comprises multi-stage amplifiers connected in cascade to one another and an output power adjusting potentiometer for controlling the bias voltages of the amplifiers and a switching circuit.
In the conventional power amplifying circuit, when an input signal is supplied to a first stage amplifier, the input signal is amplified by this amplifier and then output through a coupling capacitor. The input signal output from the first stage amplifier is amplified in order by the second to last stage amplifiers and then supplied to, e.g., a transmission antenna. If the resistance of the potentiometer varies in order to adjust output power, both a power source voltage applied to a load circuit of each of the amplifiers and a bias voltage applied to each of bias circuits vary and thus the amplification factor of each amplifier varies, thereby causing the output voltage of the input signal to be changed.
The power source voltage of a power amplifying circuit of a low power consumption type is generally low. If the bias voltages of all amplifiers are changed by a potentiometer as in the above conventional power amplifying circuit, the base potential of a transistor is lowered in the first and second stage amplifiers and the collector current of the transistor is easily affected by the voltage between the base and emitter thereof. For this reason, in the conventional amplifying circuit, the output power is susceptible to variations in temperature and the temperature characteristics of the circuit are deteriorated.